Electron beam furnace



SS1 REFEENCE R gig-LN].

May 5, 1964 A. o. DU BOIS ETAL 3,132,198

ELECTRON BEAM FURNACE 3 Sheets-Sheet 1 Filed Jan. 15, 1962 ANOPia/ 0,0050/5 (#9255 AA HAM .5

INVENTORS y 1964 A. o. DU BOIS ETAL 3,132,198

ELECTRON BEAM FURNACE 3 Sheets-Sheet 2 Filed Jan. 15, 1962 67/4 455 1d,fln/vzs INVENTORS y 1964 A. o. DU BOIS ETAL 3,132,198

ELECTRON BEAM FURNACE Filed Jan. 15, 1962 3 Sheets-Sheet 3 Awaeia/ 0,9030/5 Hon 4P0 f/ZEKF? 67/49:!{ &5 F I 6 4 United States Patent Ofitice3,132,198 Patented May 5, 1964 3,132,198 ELECTRON BEAM FURNACE Andrew 0.Du Bois, El Cerrito, Howard R. Harker, Pleasant Hill, and Charles W.Hanks, Orinda, Calif., assignors, by mesne assignments, to StaufferChemical Company, New York, N.Y., a corporation of Delaware Filed Jan.15, 1962, Ser. No. 166,119 7 Claims. (Cl. 139) This invention relates toan electron beam furnace, and, more particularly, to an electron beamfurnace in which materials are heated to melting and in which theelectron beam sources are isolated from materials being melted in orderto minimize the risk of gases which are frequently released and ionizedduring the melting, being attracted to the beam sources to establish ashort circuiting flow of electrons.

In an electron beam furnace, materials are bombarded by high-energyelectrons which are projected in a beam from sources called electronguns. The materials 'may be positioned or fed into the furnace to atarget zone overlying the open top of a crucible or mold where thematerial is melted and caused to drop downwardly into the mold.Preferably, the top of the material in the mold is itself heated inorder to maintain a molten pool of metal on top of the ingot so that, asthe lower portion of the ingot cools, it is built up uniformly.

As the raw material is melted, quantities of gas are unavoidablyreleased and, under the continuing bombardment of the electron beam,ions of the gas are formed which are susceptible to attraction by thefield of electron guns. If the gas ions succeed in reaching the fieldestablished between the cathode and the anode of the electron guns, theusual result is a shorting of the electron guns by the resultant flow ofelectrons across the field. Moreover, heavy ions frequently formedduring vaporization of the material might themselves cause electron gundamage if attracted at high velocity to the field of the gun.

The advantage of isolating the guns from the material has beenrecognized, but it is an advantage not easily realized in practice,particularly because of the difliculty of guiding high-current,high-intensity electron beams from a remote or isolated zone through asmall aperture in the shield and from there into the melting zone andonto the target with any degree of accuracy.

It is, therefore, an object of this invention to provide an electronbeam furnace including means for isolating the electron source from thegases and vapors released during melting.

It is a further object of this invention to provide means for guiding anelectron beam from an isolated source.

These effects are accomplished by providing an elongate electron gunwhich produces a ribbon-shaped beam and isolating the electron gun byprovision of a shield adjacent to the gun with a slit in the shieldthrough which the ribbon-shaped electron beam is projected, the slitbeing substantially the same size and shape as the cross section of thebeam. However, this raises the further problem of reshaping the beam foroptimum melting effect.

It is a further object of this invention to provide an electron sourceisolated by a shield and conditioned to project a beam through a slit inthe shield, together with means for guiding and reshaping the beam forbombardment of material.

The last-mentioned object is accomplished by providing a magnetic fieldwith curved lines of force arching across the ribbon-shaped beam, afterit passes through the slit, whereby the beam is simultaneously deflectedin a direction perpendicular to its width and converged to a narrowwidth.

In carrying out this invention, a horizontal shield is provided withinthe main furnace envelope at or below the level of the target which isto be bombarded and heated, to isolate the electron beam target from theelectron guns. The electron guns are disposed below the shield and aimedto project their beams upward through narrow slits in the shield, andfor this purpose an elongate gun filament is provided so that the beamis also long and narrow in cross-section, or ribbon-shaped. The beam isthen placed under the influence of a magnetic field adapted to reshapeit as well as to guide it onto the target. Particularly, the magneticfield includes arching flux lines crossing the beam path above theshield and disposed with respect to the motion of the electrons, so thatthe diverting forces imparted at right angles to the lines of forceinclude converging components which act in opposition against the widthof the beam, i.e. its long cross-sectional dimension, effectively tocompress the width while permitting the beam to spread in the otherdimension. Under the influence of these arching lines of force, thecross-section of the beam gradually assumes a more nearly circularconfiguration.

The shield through which the electron beam is projected is at theelectric potential of the anode component of the electron gun anode, inorder to reduce the tendency for gaseous ions to be attracted past theshield into the field of the electron gun. Hence, after passing throughthe slit in the shield, the electrons are in an electrically field-freeregion, and undergo no appreciable change in speed, although theirdirections are altered by the magnetic field. Moreover, heavier ionsincluding those of the material vaporized during the melting process arenot influenced by the magnetic field to the same extent as are the muchlighter electrons, and are not accelerated by an electric field, theregion above the field being electrically field-free. Hence, there islittle tendency for the heavy ions to curve down from the target throughthe slits in the shield. Any ions that do get through the slits and intothe electric field of the guns move in less curved trajectories than theelectrons do, and thus do not bombard the cathode. Isolation of the gunfrom all ions within the melting chamber is enhanced by the extremelylimited access to the electron guns afforded by the narrow slit in theshielding enclosure.

Other objects and advantages of this invention will become apparent fromthe specification following when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 is an elevational view in partial section of an electron beamfurnace embodying the present invention;

FIG. 2 is a horizontal section view taken along line 2--2 of FIG. 1;

FIG. 3 is a partial horizontal section view showing the influence of themagnetic field 'on the electron beams; and

FIG. 4 is a partial vertical view showing the influence of the magneticfield in a horizontal plane.

Referring now more particularly to the drawings, the electron beamfurnace of this invention includes an outer enclosure or envelope 10containing a crucible or mold 12 which may, if desired, be formed with awater-cooled jacket 14. The mold 12 is open at the top 15, so that rawmaterial 16 above the mold may be melted at 16a to drip into the mold orcrucible and form a molten pool 17 on the top of the ingot 18 into whichit is ultimately solidified and withdrawn through an opening 20 in thebottom of the furnace 10 by any suitable means (not shown).

The raw material to be melted may be fed by any suitable means, whichare here shown merely for purposes of schematic illustration, as feedrollers 21, in order progressively to bring the end portion to a targetor melt zone overlying the open top 15 of the mold 12. There, it issubjected to bombardment and progressively melted by a series ofelectron beams 24 projected from a plurality of electron guns 25, eachinfluencing a filament 26, a cathode 27 and an anode 28. The filament 26may be an elongated rod or, as shown in FIG. 2, it may be formed ofelongated hairpin configuration. Whatever its specific form, thefilament 26 is designed to project a thin but wide electron beam.

Straddling each electron gun 25 is a generally U-shaped magnet 30including a coil 31 and pole pieces 32 which converge progressivelytoward their outer ends, as shown in FIG. 2, for a purpose hereinafterto be described.

As a particular feature of this invention, a floor or shield 40 isprovided around the crucible 12 above the level of the electron guns 25in order to isolate the guns from the target zones, at the end of 16a ofthe raw material and in the open top of the crucible. From both of thetarget zones gases may be released during melting and could short outthe electron guns if the ions thereof drift into the field of the gun toset up a flow of electrons. The shield 40 is provided to preventinvasion of the electron guns by such gaseous ions. If desired, theshield 40 could be surrounded by a plurality of heat radiating panelswhich afford a labyrinth path for removal of gases while minimizing heatlosses by radiating heat back toward the source at the center of thefurnace. Here, there is shown simply a single vertical panel 41 forminga partial heat shield which also further isolates the electron guns fromthe source of released gases. The floor 40 and panel 41 are formed ofnon-magnetic material, such as copper, so as to afford no substantialeffect upon the magnetic field generated between the pole faces 32.Further, the floor or shield 40 is maintained at the potential of theanode 28, as by means of a mounting strap 43 to form a barrier togaseous ions that might otherwise be attracted to the field of theelectron guns. Both the electron gun and the shield 40 may be energizedfrom a conventional power supply 42.

Provided in the floor or shield 40 above each electron gun 25 is anarrow slit 44 through which the electron bears are aimed, theirparticular size and location being closely controlled in View of theelectron velocities and the influencing magentic field. For maximumisolation of the electron guns 25, it is, of course, desirable to makethe slits as narrow as possible and, in conjunction therewith, thefilaments 26 of the electron guns are of elongate configuration .so thatthe electron beam issuing from each gun is of a long and narrowcross-section aimed to pass freely through the appropriate slit 44.

With particular reference to FIG. 3, after the electron beam passesthrough the slit 44 and arches over toward the mold 12, it moves abovethe level of the pole faces 32 and the magnetic field through which ittraverses comprises progressively higher arching lines of force 33. Thisis shown particularly in FIG. 3 and becomes more pronounced in the fluxflow between the tops and remote sides of the magnet pole pieces 32. Inconjunction with this arching magnetic field, there is a substantial andprogressively increasing electron velocity vector directed into theplane of the paper in FIG. 3. As to this electron velocity vector, thedeflecting forces acting on the electrons under the influence of amagnetic field moving in an arcuate path from right to left, being atright angles to the field, tend to converge as indicated by the arrowsF, and approach a state of direct opposition at the extremities of thebeam width adjacent to the pole pieces. These converging deflectingforces, acting in opposition, tend to compress the beam and reduce thewidth thereof. Moreover, since the deflecting forces are at right anglesto the electron velocities, as well as the magnetic field direction, thecompression deflecting forces are further intensified by the convergingpaths of the electrons. The strength of the deflecting forces continuesthrough the outward trajectory of the electron beam because theconverging pole pieces 32 generate a field between them which is ofgradually increasing strength toward their ends to compensate for thereduction in field strength due to vertical distance from the polepieces.

More over, with reference to the top view of FIG. 4, the converging polepieces also generate arcuate lines of force between them which, actingon the vertical velocity vectors of the electrons, provide convergingdeflecting forces further to compress the width of the beam. Outward ofthe ends of the pole pieces indicated by the plane A the field lines offorce arch outwardly of the furnace 10. Consequently, the deflectingforces at right angles to these lines of force, considered with thevertical velocity vectors of electrons emerging from the plane of thepaper in FIG. 4 are also converging as shown by the arrows F. Then, ifthe electron velocity and strength of the magnetic field is adjusted sothat the electron beam arches over beyond the ends of the pole pieces 32and commences its downward trajectory toward the mold 12 inward of theplane A the deflecting forces continue to converge because the lines offorce beyond the ends of the pole pieces commence arching inward of thefurnace. Therefore, relative to the direction of the electron paths, themagnetic field arches in substantially the same direction. As shown inFIGS. 3 and 4, the magnetic field is barrellike in configuration and theelectrons projected from the guns 25 travel within the barrel.Therefore, throughout the electron trajectory upward between theconverging pole pieces, across above the pole pieces and downward beyondthe ends of the pole pieces the magentic field continuously arches overthe paths of the electrons in the same relative direction, withresultant converging deflecting forces. This barrel configuration can beenhanced by rounding the ends of the pole pieces 32 as shown in FIG. 1so that the flux path emerging perpendicular to the edge of the polepieces arches both inwardly and upwardly. Since the electrons follownaturally diverging paths with respect to the other cross-sectional axisof the beam, the beam is spread in the plane of FIG. 1 and compressed indirections parallel to the planes of FIGS. 2 and 3. As a result, thecross-sectional dimensions of the beam are substantially equal alongboth axes when the beam enters the crucible.

Vapor ions formed during vaporization such as those of the metal itself,are of course not influenced by the magnetic field to nearly the sameextent as are the much lighter electrons. Consequently, there is littlelikelihood that such relatively heavy particles would be influenced bythe magnetic field to traverse the reverse path of the bombardingelectron beams and move from the melt zones into the electron guns.Further, the extremely limited access to the zone of the guns providedby the narrow slits sharply reduces the possibility of vapor moleculedeposition of the electron gun components.

In direct communcation with the main furnace envelope 10 through largewindows or ducts 48 are evacuation chambers 50 through which theenvelope 10 is continuously evacuated by vacuum pumps 52 to a pressurein the order of one micron of mercury. Similarly, large evacuationwindows 56 are formed in the heat radiating panels 41 and they arearranged in substantial alignment with the main furnace evacuationwindows 48 so that direct paths of gas and vapor evacuation are providedto the evacuation chambers 50. As a result, the possibility that gas orvapors above the shield 40 will drift around and below the shield to theelectron guns 25 is extremely remote. Preferably, the windows 56 of theinner panels 41 are provided with a series of vanes or baflles 58 toprovide vapor condensation surfaces and heat conserving radiationmembers.

If desired, the shield or floor 40 and the pole faces 32 are themselveswater-cooled to protect them against the intense heat within the furnaceenclosure 10 resulting from electron bombardment. This water-cooling maybe accomplished by securing copper tubing or the like (not shown) tothese elements.

Other modifications and changes in and to the preferred embodiment shownmay be made by those skilled in the art, without departing from thespirit and scope of this invention which is defined by the claimsappended hereto.

What is claimed as invention is:

1. An electron beam furnace comprising a furnace enclosure,

an electron gun including an elongate thin filament for projecting ahigh-energy electron beam having a long and narrow cross section upwardin said enclosure,

means for presenting material to be treated to a melt zone within saidenclosure,

a horizontal shield disposed adjacent to and above the gun between saidelectron gun and said melt zone, said shield including a long, narrowslit therethrough through which said electron beam is aimed, and

a generally U-shaped magnet having converging pole pieces on oppositesides of said electron gun so that the rearwardly and upwardly archinglines of force tend to exert converging deflecting forces against thewidth of an electron beam moving upward and forward therethrough afterit passes through said slit and to guide the beam in a curved path ontothe material to be treated.

2. The electron beam furnace defined in claim 1 wherein said pole piecesterminate below the point in said electron beam at which downward travelthereof commences so that lines of force arching outwardly and forwardlyfrom the ends of said pole pieces exert converging deflecting forcesagainst the width of said electron beams.

3. An electron beam furnace comprising a furnace enclosure,

a holder for material to be bombarded in the upper portion of saidfurnace enclosure,

an open top crucible in said furnace enclosure for melting material,

an electron gun in the lower portion of said furnace enclosuregenerating an electron beam of long and narrow cross-section,

a horizontal wall surrounding the open top of said crucible andisolating the lower portion of said furnace enclosure including saidelectron gun from said upper portion,

said horizontal wall having a narrow slit extending therethrough ofsubstantially the same cross-sectional dimension as that of saidelectron beam,

means generating a magnetic field for focusing said beam through saidslit and additionally compressing said beam above said wall in the longcross-sectional dimension thereof while guiding the electron beam incurved paths to bombard said material, whereby the beam cross-section atbombardment is substantially circular, and

high vacuum evacuating means in direct communication with said furnaceenclosure both above and below said horizontal wall for maintaining thefurnace enclosure at a high vacuum.

4. The electron beam furnace defined in claim 3 including bafiles insaid furnace enclosure between said crucible and said evacuating means.

5. An electron beam furnace comprising a furnace enclosure,

a means for delivering material to be bombarded to a melt zone in theupper portion of said furnace enclosure,

an open top crucible for melted material in the lower portion of saidfurnace enclosure,

a plurality of electron guns in the lower portion of said furnaceenclosure,

a horizontal wall surrounding the open top of said crucible and dividingthe main furnace enclosure above said electron guns,

said horizontal wall defining a plurality of long, narrow slitstherethrough,

each of said electron guns being aimed to project a high-energy beam oflong and narrow cross-section upward through a separate one of saidslits,

magnetic field generating means establishing a magnetic field exertinglaterally inward deflecting forces against the long cross-sectionaldimension of said beam while guiding said beam in a curved path tobombard the material, and

high vacuum evacuating means in direct communica tion with said furnaceenclosure both above and below said horizontal wall.

6. The electron beam furnace as set forth in claim 3, additionallydefined by the means generating said magnetic field comprising,

a pair of magnet pole pieces disposed below said horizontal wall withone pole piece at each end of said electron gun and establishing asubstantially uniform magnetic field in the immediate vicinity of saidguns,

and said pole pieces having ends converging toward each other towardsaid crucible for maximizing the focusing of electrons therein.

7. An electron beam furnace comprising,

a furnace enclosure,

means disposing a material for treatment in a melting zone within saidfurnace,

a horizontal wall separating a portion of said enclosure from theremainder of the furnace including said melting zone,

said horizontal wall having at least one narrow elongated slittherethrough,

at least one electron gun disposed beneath and on the opposite side ofsaid wall from said melting zone and having an elongate filament forgenerating a ribbon-shaped beam having substantially the samecross-sectional dimensions as said wall slit,

said electron gun being laterally displaced with respect to said wallslit,

a magnet field generator establishing a magnetic field having lines offorce substantially parallel to said electron gun filament in thevicinity thereof for guiding said electron beam in a curved trajectorythrough said slit for maximized protection of the electron gun from ionbombardment,

said magnetic field generator further establishing said magnetic fieldwith lines of force in a barrel shape curving over at least a part ofsaid melting zone and concave with respect to said melting zone tolaterally converge and longitudinally extend the beam cross-section sothat the beam impact pattern is substantially circular for maximizedbeam heating efiiciency,

and evacuation means disposed in direct communication with said furnaceenclosure both above and below said horizontal wall for continuouslymaintaining the furnace enclosure at a high vacuum.

References Cited in the file of this patent UNITED STATES PATENTS2,252,052 Van Embden Aug. 12, 1941 2,291,948 Cassen Aug. 4, 19422,715,693 MacNeille et a1 Aug. 16, 1955 3,046,936 Simons July 31, 19623,068,309 Hanks Dec. 11, 1962

1. AN ELECTRON BEAM FURNACE COMPRISING A FURNACE ENCLOSURE, AN ELECTRONGUN INCLUDING AN ELONGATE THIN FILAMENT FOR PROJECTING A HIGH-ENERGYELECTRON BEAM HAVING A LONG AND NARROW CROSS SECTION UPWARD IN SAIDENCLOSURE, MEANS FOR PRESENTING MATERIAL TO BE TREATED TO A MELT ZONEWITHIN SAID ENCLOSURE, A HORIZONTAL SHIELD DISPOSED ADJACENT TO ANDABOVE THE GUN BETWEEN AND ELECTRON GUN AND SAID MELT ZONE, SAID SHIELDINCLUDING A LONG, NARROW SLIT THERETHROUGH THROUGH WHICH SAID ELECTRONBEAM IS AIMED, AND A GENERALLY U-SHAPED MAGNET HAVING CONVERGING POLEPIECES ON OPPOSITE SIDES OF SAID ELECTRON GUN SO THAT THE REARWARDLY ANDUPWARDLY ARCHING LINES OF FORCE TEND TO EXERT CONVERGING DEFLECTINGFORCES AGAINST THE WIDTH OF AN ELECTRON BEAM MOVING UPWARD AND FORWARDTHERETHROUGH AFTER IT PASSES THROUGH SAID SLIT